Method of nitriding steel articles



Dec.. 29, 1931. R. SERGESON 1,333,195

7 METHOD OF NITRIDING STEEL ARTICLES Filed Sept. 10, 1929 l,

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gwventoz Eobert Seryeson Patented Dec. 29, 1931 UNITED STATES PATENT OFFICE ROBERT SERGESON, OF MASSILLON, OHIO, ASSIGNOR, BY MESNE ASSIGNMENTS, TO BE- PUBLIC STEEL 'CORPORATION, OF YOUNGSTOWN, OHIO, A CORPORATION OF NEW JERSEY METHOD OF NITRIDING- STEEL ARTICLES Application filed September 10, 1929. Serial No. 891,561.

This invention relates in general to the intriding of alloy steel articles and more particularly to a novel method of nitriding such articles, whereby the time of nitriding is materially lessened Without at the same time decreasing the surface hardness of the nitrided zone thereof.

According to Fry, a well-known investigator of the nitriding process, no nitriding should be done at a temperature in excess of 1075 'F., else the marginal layers of the nitrided zone will be brittle and tend to peel off. For this reason, most nitriding has heretofore been done below 1000 F., and while,

,- in most instances, the nitrided zones have been found to be extremely hard, the zones are relatively thin and the time required is often as much as 100 hours.

It has been proposed to decrease the time required for nitriding by nitriding at temperatures above 1000 F. and up to as high as 1250 F. Increasing the temperature of nitriding in this manner, while it results in a material lessening of the time required for producing a nitrided zone, results also in a decrease in surface hardness of such zone. It has been proposed to decrease the time required for nitriding without sacrificing surface hardness by employing a duplex heating cycle in which the nitriding is conducted at a relatively high temperature for a certain number of hours and then at a relatively low temperature for a like or different number of hours. This method results in only a slight increase in surface hardness.

As the result of a series of experiments, I have found that by reversing the sequence of temperatures in the aforesaid duplex heating cycle, i. e., by nitriding at a relatively low temperature for a certain number of hours and then at a relatively high temperature for a like or different number of hours, the surface'hardness of the nitrided zone is increased to. a degree comparable with that of articles nitrided at comparatively low temperatures for long periods, and at the same time, the toughness of the nitrided zone is increased so that it is enabled to withstand greater deformations without peeling, chipping or spalling thereof.

I attain these results by nitriding the articles at a temperature of from about 850 F. to about 1000 F. for a period varying from about 5 to about 40 hours, and then at a temperature of from about 1100 F. to about 1250 F. for a period varying from about 5 to about 40 hours.

As an example of my novel method of nitriding, attention is directed to the curves shown in the accompanying drawing, wherein the effects of nitriding by the two aforesaid duplex heating cycles are clearly shown.

The specimens employed in obtaining the data for these curves, were identical in size, sha e and composition, their analysis being as ollows:

Per cent Carbon 315 Molybdenum -Q 19 Silicon 15 Chronium 1. 68 Manganese 63 Aluminum 70 The specimens, designated B were nitrided at a temperature of 11-80 F. for 15 hours and then at a temperature of 950 F. for 15 hours.

- The specimens, designated A were nitrided at a temperature of 950 F. for 15 hours and then at a temperature of 1180 F. for 15 hours.

The curves, which represent the averages of the values obtained for hardness, clearly show that the surface hardness of the specimens designated A is considerably higher than that of the specimens designated B and that the hardness values for depths up to .015 in. are higher than those of the latter specimens.

Claims 1. The method of nitriding alloy steel articles which consists in nitriding the articles at a temperature of from about 850 F. to about 1000 F. for from about 5 to about 40 hours, and then at a temperature of from about 1100 F. to about 1250 F. for from about 5 to about 40 hours.

2. The method of nitriding alloy steel articles which consists in nitriding the articles at a temperature of'about 950 F. for about 15 hours, and then at a temperature of about 1180 F. for about 15 hours.

' In testimony whereof I aifix my signature.

ROBERT SERGESON. 

